WO 2007/093572 A1 describes a device for mounting a protector on a DIN rail, comprising a fixed locking projecting portion fixedly formed on one of opposite sidewalls of a fitting recess portion of a base unit forming the protector into which recess portion the DIN rail is fitted, and a movable locking member disposed on the other of the opposite sidewalls and which is movable in a direction in which the movable locking member approaches or leaves the fixed locking projecting portion, wherein the protector is mounted on the DIN rail by locking an upper-end locking vertical portion of the DIN rail on the fixed locking projecting portion, then moving the movable locking member in the direction in which the movable locking member leaves the fixed locking projecting portion, then fitting the DIN rail into the fitting recess portion, and thereafter moving the movable locking member closer toward the fixed locking projecting portion.